1. Field of the Invention
This invention relates generally to a method for optical fiber bonding. More particularly, the present invention relates to a method for bonding an optical fiber directly to a substrate. The inventive direct bond includes only the optical fiber and the substrate; no other adhering or connection components are required.
2. General Background and State of the Art
In recent years, there have been significant developments in the field of optical communications. For example, while a decade ago, communications lines transmitted data in a single wavelength signal, today's scientists are able to transmit 80-160 colors at once through a single fiber. Not only has the amount of data that can be transferred increased, but, the speed at which this data is carried has also increased, with reports of speeds as high as 10 gigabits per second.
It is anticipated that data transfer by existing electrical computing systems using metallic connections will be replaced by totally optical systems using light impulses. Optical technologies are already widespread in such areas as medical equipment. For example, surgeons are now capable of doing non-invasive heart bypasses from a remote location using optical equipment. The optical link provides the necessary connection speeds to make this possible. Currently, the optical fibers to emitter connections are made with mechanical couplers, or micro-machined sleeves containing fibers inserted into holes using an adhesive. Unfortunately, these connections require manual processes that will prohibit making these connections on a large scale. Developing a new and improved optical fiber bonding process will translate to faster, more efficient and cost-effective technologies. However, these developments have been quite limited and hindered, for a variety of reasons.
While generating electrical impulses for high-speed metal connections has been developed for years and is widely used, optical systems require light impulses rather than electrical impulses. These light impulses must be generated at a very high speed in order to sustain reliable transfer of data over glass fibers in totally optical systems. Other considerations and limitations in light generation include reliability, size, efficiency and cost. In view of these considerations semiconductor lasers are ideal candidates for the task.
Semiconductor lasers are becoming the industry choice for light generation in optical data transfer. Existing optical technologies are being developed and used in many systems including telecommunication switching systems, avionics devices and communication between supercomputers and mass data storage devices. Additionally, optical technology is currently being tested for other applications including totally optical personal computers.
To date, there are two main types of semiconductor lasers: in plane, edge emitting lasers (IPLs) and out-of-plane, vertically emitting lasers. IPLs employ reflection of light back and forth in the plane of the wafer that they are produced on. IPLs are used extensively and can be very effective in some applications. Vertical Cavity Surface Emitting Lasers (VCSELs) are semiconductor lasers that emit light normal to the plane of the wafer.
There are many advantages of vertical emission lasers when compared to IPLs. For example, VCSELs are ideal for large scale array fabrication. In contrast to IPLs, where the wafer must be cleaved before the array is functional, VCSELs propagate light out of the plane of the wafer so arrays of lasers can be immediately fabricated. Additionally, each individual VCSEL can be pre-tested on the wafer prior to the expensive and difficult task of separating the wafer into microchips. Another advantage of VCSELs is that defective lasers are immediately detected and discarded without expending additional time and money associated with the separation process.
Another advantage of VCSELs is the fact that they have a circular shape and contain low-numerical aperture output beams. This enables the output to be coupled to optical fibers with ease and with a high coupling efficiency. In contrast, IPLs emit a wide elliptical beam that expands quickly as it moves away from the laser, making it difficult to achieve high coupling efficiency with optical fibers.
Also, IPL beams require additional optical components to couple the output to a fiber, and additional components are expensive and cumbersome to incorporate into a system. Moreover, the vertical nature of VCSEL output yields less stringent fabrication tolerances. For example, variations in fiber thickness are not a problem using VCSELs because of the vertical output nature, while thickness variations affect alignment of fibers in IPLs. This is a significant advantage of VCSELs, since variations in thickness of oxide growth layers on the wafer can occur during fabrication. Lastly, in contrast to IPLs, which need to be placed on the edge of a microchip, VCSELs can be placed at any location on a microchip. Since IPLs have to be on the edge of a microchip, they effectively increase the overall size of an array.
Thus, IPLs limit the flexibility of integrating semiconductor lasers into all types of components. However, vertical emission lasers (VCSELs ) can be incorporated into a wide range of products including totally optical personal computers. Moreover, VCSELs are fabricated in place on the same wafer as the electrical circuitry, enabling “one-step” fabrication of complete systems on a single microchip. Using the technology developed for producing the integrated circuit, fabrication techniques allow designers to incorporate the semiconductor lasers and the electronics that control them. The electronics are fabricated using photolithography and thin film metal deposition steps. The lasers are fabricated by growing oxide layers and deposited material on the same chip in a temperature and humidity controlled environment.
In addition to suitable semiconductor lasers, advances in data transmission using optical systems also require that the lasers are coupled to optical fibers. Achieving a suitable and reliable coupling has proved difficult. Holm et al., have developed a silicon coupler mechanism for coupling fibers to VCSELs by etching 125 micron holes in the silicon wafer using deep reactive ion etching (DRIE) (Holm, J. et al., 2000, Actuators, 82,245-248). The DRIE method is a micro-machining process that makes holes that are conical shaped, with a slightly larger diameter at the bottom of the wafer. Correspondingly, a 125-micron diameter fiber is inserted and glued to the 125-micron circular shaped bottom of the silicon wafer. The conical shape acts as a guide while the exact 125 micron diameter ensures accurate alignment of the fiber. Then using standard surface micro machining, electrodes and solder pads are produced on the top surface of the silicon wafer. The electrodes facilitate the power to the semiconductor lasers and the solder pads mount the lasers themselves onto the wafer.
Unfortunately, a significant drawback of Holm's silicon coupler is its manner of fiber placement. Most laser devices contain mirrors. In order to avoid damaging these mirrors, the fibers have to be inserted prior to mounting the lasers. However, because fibers protrude out, insertion of fibers prior to mounting the laser becomes difficult. Conversely, if the laser is mounted first, then extreme care and precision must be used to insert the fiber. To resolve this obstacle, Holm et al., bond a silicon “lid” to the top surface of the silicon wafer. This lid contains etched holes of 50-micron diameter, located in the center of the 125-micron holes on the silicon wafer. Again, the DRIE micro-machining process performs etching. The lid functions as a stop for the 125-micron diameter fiber. The lid also provides a more stable system by allowing the solder pads to be placed closer to the center of the laser. Unfortunately, a significant disadvantage of this modification is that the aperture of the fiber is now limited because it is partially covered by the lid.
Another problematic issue is the manual process of inserting and gluing the fibers in place on the silicon wafer. Ideally, if optical systems are to be used commercially, the system must be efficient and automated. Manual insertion and gluing of the fiber to the wafer is cumbersome and inefficient. Furthermore, an automated optical system must also be able to connect and communicate quickly and precisely with other components of that optical system.
Another problem with this method involves the couplers required to attach the optical fiber to the silicon wafer. In order to harness the output of the laser with the fiber using silicon couplers additional fabrication steps are necessary. For example, the difficult task of fabricating the silicon coupler must be completed. Then the lasers need to be fixed to the silicon coupler. Fixing the laser to the coupler requires separating the lasers on the wafer, placing them on the coupler and heating and cooling the solder to adhere the lasers to the coupler; all without damaging the lasers. Finally, the fibers have to be inserted and glued in position into the silicon coupler.
Additionally, the Holm's silicon coupling device has an optical coupling efficiency of ˜90%. The 10% loss is mainly attributed to the free space between the laser and the fiber. Hence, reducing the gap should increase the coupling efficiency. So, either smaller gaps need to be in place or, ideally, the gap is eliminated entirely by bonding the fiber directly to the laser. However, reducing the gap remains a challenging problem.
What is needed is process in which a semiconductor laser in a silicon wafer is coupled to a fiber in a one step process performed directly on the silicon wafer. Such a one-step process would eliminate the need to separate laser arrays during fabrication, thereby reducing cost and unnecessary steps. What is also needed is a process for coupling the laser to the optical fiber such that the resultant communication is highly efficient and reliable.